There are many settings within water delivery systems in which the amount of water that must be contained varies over time. For example, water expands when heated. In a closed system, this expansion may cause dangerous increases in water pressure. While water heaters have relief valves to vent excess pressure and prevent damage to the water heater and surrounding piping, it is undesirable to have hot water venting out of a tank in a residential setting. As a result, expansion tanks are used to absorb the excess pressure and release water back into the water heater when the pressure decreases. In addition, expansion tanks may be used to modulate pressure spikes in water systems in which pressure is supplied by a pump, e.g., domestic well systems. Expansion tanks also find applications in forced hot water heating systems, water towers, and other settings.